1. Field of the Invention
The invention relates generally to optical fibers. More specifically, the invention relates to an optical sensor that employs backscattering to determine a location in the fiber sensor.
2. Discussion of the Related Art
Optical fibers have been employed in sensing applications, including measurements of distributed strain and temperature in an optical fiber as well as acoustic signals impinging on the fiber. In one sensing application, an optical signal is transmitted into the optical fiber and perturbations in the fiber core(s) result in back scatter which may be analyzed to obtain the shape of the fiber. In another, the distributed scattering signal provides a measurement of acoustic waves that act to strain the fiber. In such distributed sensors, measurements may be performed at any part of the array. Thus continuous measurements with a certain spatial resolutions are possible along the distributed sensor.
One type of back scattering that has been proposed for use in fiber shape measurements is Rayleigh back scattering. Unfortunately, the spectral and spatial response of Rayleigh back scattering is random. Further, localization of the signal requires precise knowledge of a group index of a fiber. This limits the reliability of Rayleigh back scattering in distributed sensing applications. If there is more than one core in the optical fiber, an additional problem arises. It is necessary to correlate the Rayleigh back scattered signal from the multiple cores in order to obtain an accurate estimate of the fiber shape. However, this also requires accurate knowledge of the group index variation between as well as along the different cores. More particularly, the task of reconstructing the shape requires a measurement of the exact position from backscatter in the fiber. Typically, this will depend on the effective index of a mode propagating in a core of the fiber and, in particular, on the group index of the propagating mode. The group index permits a conversion of temporal and frequency domain information obtained into spatial domain information. However, the effective index and group index will not be the same in all cores of a multicore fiber. Moreover, the effective index and group index will not be the same for both polarizations.
Related art techniques to overcome this problem require accurate calibration of the propagation of light in each core of a multicore fiber. Unfortunately, any drift in waveguide properties can render inaccurate any such calibration. Another related art technique involves the use of fiber gratings to provide the scattering. However, this technique requires the inscription of fiber gratings along the entire length of the fiber. Such fabrication adds cost and complexity to a fiber sensor.
Thus, a need remains in the art to improve measurements of distance along an optical fiber using optical backscatter that does not require continuous fiber gratings or absolute calibration stability.